Minor bubbling on rocker

My 72 Duster is pretty solid, but not rust free. Its not showy and thats what I wanted this time. Its had some moderate panel patches (lower quarter and taillight panel) and I see some minor bubbles forming on the rockers- mainly just behind the back of the door. My question is that I'd like to slow down the rust process, and from what I understand, it would be good to get something to seal up the inside of that rocker. Once done, I'll touch up the bubbles on the outside to seal it all up. I've seen some sprays that are advertised for these boxed in areas, and this Eastwood stuff sound perfect (https://www.amazon.com/dp/B003NTPE9W/?tag=joeychgo-20) but they want around $28 for a 14oz can. I have used Rustoleum Rusty Metal Primer in the past and felt like it maybe helped, and could probably spray it inside the rocker? I know some of the rust proofers drill a small hole under the sill plate, I cant remember if our rockers have openings on the bottom like I've seen on other cars, but I may be able to squirt some through them if so and not create an access hole on the top. I've also seen Rust Bullet etc, they seem to be fairly expensive too? ANY and ALL ideas and suggestions appreciated!